1. Field of the Invention
The present invention relates to a process for treating materials to improve their structural characteristics. This process may be used with particular advantage to treat metals used to make tools, such as drill bits and cutting knives, that are subject to wear. This process may be used with equal advantage to treat metals such as welding tips that are consumed when used. In both cases, metals treated with this process wear or are consumed at substantially lower rates than do the same metals when not treated.
The process of the subject invention may also be used to treat materials other than metals to improve their structural characteristics so as to provide longer useful life.
2. Description of the Prior Art
Many processes for treating metals and other materials are now known. For example, U.S. Pat. No. 3,891,477 (Lance et al.) relates to a treatment process that uses cryogenic cooling and specifically includes the step of incrementally cooling a material from normal room temperature to about -320.degree. F. by supporting the material above a body of liquid nitrogen and lowering the material toward the liquid nitrogen at a rate less than that at which thermal fracturing occurs. The rate of temperature reduction is effected in at least two stages above and below a predetermined thermal fracture threshold for the material. The material is thereafter immersed into the liquid nitrogen for a period within the range of about 18 hours to about 30 hours. When the immersion step is complete, the material is separated from the liquid nitrogen and permitted to return to room temperature.
Thus, the process described in the Lance Patent involves lowering the temperature of the material at two separate temperature reduction rates and immersing the material in liquid nitrogen for an extended period of time, namely about 18 to about 30 hours. The Lance Patent claims that these controlled steps, including the extended immersion of the material in the liquid nitrogen, eliminate the need for a heating step or steps. Moreover, according to the Lance Patent specification, other processes, in which a material is cooled to sub-zero temperatures and subsequently heated to elevated temperatures in the range of 300.degree. F. to 500.degree. F., merely provide surface hardness rather than a uniform refined microstructure throughout the treated material.
U.S. Pat. No. 3,819,428 (Moore) relates to a metal hardening process in which a metal is first heated in two stages up to a temperature in the range of 750.degree. C. to 1325.degree. C. (1382.degree. F. to 2417.degree. F.) for a short period, quenched, and then cooled to a temperature of about -80.degree. C. to -120.degree. C. (-112.degree. F. to -184.degree. F.) by direct contact with cold gas or indirect contact with cold gas or cold liquid. The cooling step in the example given in the Moore Patent was performed for about twenty minutes. After the cooling step, the material is subjected to a final single tempering within the range of 150.degree. C. to 600.degree. C. (302.degree. F. to 1112.degree. F.).
The Moore Patent specifies that in the cooling step, temperatures below -120.degree. C. (-184.degree. F.) and direct contact of a treated metal with cold liquid are to be avoided. Both occurrences are said to cause undue stress or impose severe strain on the metal structure.
Still other similar processes are known. As an example, U.S. Pat. No. 2,197,365 (Kjerrman) relates to a method of hardening a steel article by subjecting it to successive heating, cooling, and heating steps. The initial heating step is to about 850.degree. C. (1562.degree. F.); the intermediate cooling step is ultimately to about -78.degree. C. (-108.degree. F.); and the subsequent heating tep is to about 200.degree. C. (392.degree. F.).
As a further example, U.S. Pat. No. 2,958,617 (Perry) relates to a method for hardening stainless steel by initially heating it to about 1700.degree. F. to 1850.degree. F. and subsequently cooling it to about 850.degree. to 950.degree. F., forming the steel into a part, and reheating it to 950.degree. F. to 1150.degree. F. and then refrigerating it at about -30.degree. F. to about -150.degree. F. Ultimately, the steel is reheated to 700.degree. F. to about 1150.degree. F.
U.S. Pat. No. 3,185,600 (Dullberg) relates to a method of hardening metal that includes heating the metal by a solution heat treatment and thereafter introducing the metal at the temperature of the solution treatment directly into a quenching medium to reduce its temperature rapidly to below about -50.degree. F.
U.S. Pat. No. 2,990,275 (Binder et al.) relates to a hardenable stainless steel which includes an aggregate of cobalt and nickel sufficient to impart a substantially austenitic structure at temperatures above about 950.degree. C. (1742.degree. F.).
Still another process for treating a metal is described in U.S. Pat. No. 2,624,688 (Svensen) and involves preheating the metal to 1250.degree. F. to 1300.degree. F., rapidly heating the metal to a hardening temperature of 1575.degree. F. to 1600.degree. F., and then quenching the metal to no less than 100.degree. F. to 110.degree. F. The metal is then tempered at 200.degree. F. and subsequently subjected to cold treatment at -125.degree. F. to -150.degree. F. Then the metal is finally subjected to another tempering, cooling, tempering cycle.
U.S. Pat. No. 2,949,392 (Willey) discloses a method for relieving residual stresses in light metal articles that includes cooling the article to temperatures below -100.degree. F. and as low as -425.degree. F. for short periods of time and than subjecting the articles to a current of condensable vapor at a temperature not lower than the boiling point of water.
Materials Improvement, Phoenix, Ariz. has offered a commercial material treatment process in which metals are cooled in a dry atmosphere to temperatures below -300.degree. F. for periods in excess of 24 hours and below -100.degree. F. for 60 hours. The long period of exposure to cold temperatures is claimed to result in a more uniform, refined microstructure with greater density in a stabilized form of the metals. After the cold treatment the material returns to room temperature and is then heated to about 300.degree. F. The Materials Improvement process has the disadvantage of requiring expensive refrigeration equipment to create the cold, dry atmosphere and taking a long time to complete.